Celtic captain Scott Brown could miss three months of action as a result of knee ligament damage.
Midfielder Brown twisted his knee in a challenge towards the end of Celtic's 0-0 draw with Kilmarnock on Saturday and the Scottish champions are waiting for scan results to see how serious the injury is.
Celtic manager Ronny Deila told reporters ahead of Thursday's Europa League clash with Ajax at Parkhead: "It was very unfortunate in a challenge near the end of the game. He twisted it and got an injury on the outside ligament.
"We hope that an operation is not necessary, we are waiting for the experts to look at the pictures so we can be even more sure what to do, if it is going to be an operation or not.
"It is always tough to lose your captain, but having said that we have done that before and the last period he was out we won a lot of games."
As well as Brown, Celtic are without Kris Commons, Nir Bitton and Stefan Johansen, who are all injured, with Deila hinting Scott Allan may be selected in midfield as a result.
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